Condensation Of Urea Research Articles

Anatase TiO₂-Decorated Graphitic Carbon Nitride for Photocatalytic Conversion of Carbon Dioxide.

Three types of graphitic carbon nitride (gCN) nanosheets were derived from direct thermal condensation of urea, melamine, and dicyandiamide, respectively. As the sample (uCN) synthesized from urea exhibited porous morphology and highest surface area among other gCN, anatase TiO2 nanoparticles were then in-situ deposited on uCN via solvothermal process without further calcination. The resultant Ti/uCN_x samples remained with higher surface area and exhibited visible-light activity. The derived band structure of each sample also confirmed its ability to photoreduce CO2. XPS results revealed surface compositions of each sample. Those functional groups governed adsorption of reactant, interfacial interaction, electron transfer rate, and consequently influenced the yield of products. Carbon monoxide and methanol were detected from LED-lamp illuminated samples under appropriate moisture content. Samples with higher ratio of terminal amine groups produced more CO. The presence of hydroxyl groups promoted the initial conversion of methanol. The obtained Ti/uCN_0.5 and Ti/uCN_1.5 samples exhibited better quantum efficiency toward CO2 conversion and demonstrated stability to consistently produce CO under cycling photoreaction.

Water-assisted prepared porous carbon nitride incorporated AgBr for highly efficient photocatalytic degradation of orange G under visible light

Carbon nitride is a commonly used photocatalyst for the degradation of organic dyes. However, the high rate of charge carrier recombination limit the photocatalytic activity. Here, a series of porous carbon nitride with AgBr nanoparticles (CN/AgBr) heterojunction are fabricated by thermal condensation of urea with the water and an in situ ion exchange approach. The porous structure of CN is beneficial to improve the photocatalytic performance and enhance the absorption of light energy, which is due to the multiple reflections of light within the macropores layered structure. And the incorporation of AgBr inhibit the charge carriers recombination due to the formation of heterojunction with CN. Furthermore, the photocatalytic activity of all samples are evaluated and display excellent activity in the degradation of orange G under visible light illumination. Among them, the CN/AgBr30 perform the most degradation efficiency, with a removal ratio of up to 92% within 10 min, which is attributed to efficient charge separation and transfer. At last, a photocatalytic mechanism is put forward and explained based on the ESR experiments, free radical and hole scavenging experiments and transient response photocurrent results.

Magnetically recoverable graphitic carbon nitride and NiFe2O4 based magnetic photocatalyst for degradation of oxytetracycline antibiotic in simulated wastewater under solar light

In this work, we report the applicability of NiFe2O4 loaded graphitic carbon nitride (GCN/NiFe2O4) as a magnetic photocatalyst for wastewater treatment. Herein, GCN was prepared by thermal condensation of urea and successfully loaded with NiFe2O4 nanoparticles. GCN/NiFe2O4 was characterized by FESEM, HRTEM, IR, XRD, VSM and UV visible spectral analysis. High surface area of GCN/NiFe2O4 was suitable for adsorption coupled photocatalysis. The GCN/NiFe2O4 exhibited significant photocatalytic activity for mineralization of oxytetracycline (OTC) antibiotic under solar light. The OTC was completely mineralized in 8 h during simultaneous adsorption and photocatalysis process. The rate of OTC degradation was optimal at pH 5. GCN/NiFe2O4 possessed ferromagnetic behavior and could be easily separated by external magnetic field from waste water. The excellent activity of GCN/NiFe2O4 was due to efficient separation of photogenertaed electron and hole pair. GCN/NiFe2O4 exhibited significant recycle efficiency for ten catalytic cycles.

Preparation and characterization of expanded g-C3N4 via rapid microwave-assisted synthesis

Microwave assisted heating was successfully employed in significantly accelerated preparation of expanded g-C3N4 via thermal condensation of urea. Comparing to typical synthesis route, rapid microwave assisted heating enables not only shorten synthesis time from hours to few minutes but obtain reasonable yield of the product also. Use of ceramic kiln “hotpot” with intern covered with absorbing layer plays the key role in transformation of the microwave energy into heat. Heat energy in form of radiation from the glowing inner layer surface is focused into the internal volume of the kiln resulting in extremely fast temperature increase. Thus, microwave non-absorbing starting material is extremely fast heated. The effects of reaction conditions on the structural, morphological, optical and photocatalytic properties of prepared g-C3N4 were investigated in detail. Having similar specific surface area but much larger particles than its conventionally prepared analogue, the microwave prepared material can be classified as expanded g-C3N4. Moreover, by simply changing the reaction time, photoluminescence emission of the material can be continuously tuned covering the blue to green light region of visible light. The photocatalytic degradation of Methyl Violet 2B reveal that the adsorption capacity of samples increases with processing time while the rate constant slightly decreases. It can be ascribed to the expansion of the internal surface which results in its worse availability to the diffusion transport of the dye to active sites. It also impedes photodegradation products removal. The synthesis technique described here could be considered applicable to starting materials, which cannot be sufficiently heated by microwave absorption directly.

Enhanced photocatalytic activities of g-C3N4 with large specific surface area via a facile one-step synthesis process

Graphitic carbon nitride (g-C3N4) has been considered as a promising metal-free catalyst for a wide range of application due to its intriguing properties. However, the photocatalytic performance is restricted by the small specific surface area (SSA) and limited active sites. Here, a facile one-step method was employed to prepare thin porous g-C3N4 with large SSA by condensation of urea. During the pyrolysis process, the self-generated NH3 not only participates in the condensation reaction to obtain bulk g-C3N4, but also etches crystal lattice to form thinner nanosheets. A high SSA of 141.4 m2 g−1 for the sample synthesized in the enclosed condition (ECD-CN) is obtained, which is much larger than the SSA of 57.4 m2 g−1 for the sample synthesized in the non-enclosed condition (NCD-CN). These porous nanosheets with the enhanced SSA endow g-C3N4 more exposed active sites and shorten the carrier diffusion length. As a result, the ECD-CN sample exhibits much higher photodegradation rate on rodamine B (99.0% within 10 min) and hydrogen evolution rate (504.2 μmol h−1 g−1) under visible-light. This study provides a facile and template-free strategy in preparing efficient, sustainable and visible-light-driven photocatalyst.

Oligomer‐Incorporated Polymeric Layer Framework of Graphitic Carbon Nitride for Effective Photocatalytic Hydrogen Evolution

AbstractThis paper presents a structural design of graphitic carbon nitride (CN) for use as an active photocatalyst in H2 evolution reactions. The active photocatalyst is synthesized by subjecting a CN sample to consecutive annealing and ammonia treatments. The CN sample is produced through urea condensation. The annealing treatment destroys the polymeric CN to form a defective polymeric layer framework incorporating 1,3,4,6,7,9,9b‐Heptaazaphenalene‐2,5,8‐triamine (melem) oligomers. The annealed CN exhibits a wide absorption range in visible light because the distorted structure causes conjugation of the π‐orbital with a nitrogen nonbonding state. The subsequent ammonia treatment of the annealed CN repairs the nitrogen vacancies and bridges the polymeric framework and melem oligomers. The repaired and bridged sample (N‐aCN) provides viable pathways for charge separation and transport, and active amino sites are introduced in the sample for electron injection in order to produce H2. When Pt is deposited, the CN and N‐aCN samples catalyze H2 evolution from aqueous triethanolamine solution at quantum yields of 3.7 and 17%, respectively, under 420 nm monochromatic irradiation. The high activity of N‐aCN demonstrates that the oligomer‐incorporated polymeric layer framework effectively harvests light and mediates charge transport in CN materials for photoenergy conversion.

Structure Engineered g‐C3N4 Nano‐Sheets by Switching the Pyrolysis Gas Atmosphere for Enhanced Photo‐Catalytic Degradation

AbstractWe developed a new one step approach to synthesize g‐C3N4 nano‐sheets by direct thermal pyrolysis process of urea in NH3 atmosphere. For the first time, the influence of the preparation gas atmosphere on the composition, crystalline and polymerization degree, and the activity of the g‐C3N4 synthesized from thermal condensation of urea was investigated. Impressively, the g‐C3N4 nano‐sheets obtained under NH3 gas atmosphere exhibited much superior photo‐catalytic activities to the prepared g‐C3N4 in air or N2, and the rate of the g‐C3N4‐NH3 was about 5 times higher than that on g‐C3N4‐N2 sample. The detailed characterization analysis revealed that NH3 thermal pyrolysis atmosphere contributed to the polymerization degree and the formation of the layer with a more regular structure due to the efficiently extending of the conjugated π‐conjugative system, which was favorable to the transfer of the photo‐induced charge carriers. Furthermore, we studied in depth the structure‐performance relationship in the system, and it was found that the synergistic effect of the larger surface area, the adjusted band energy structure and the well crystallization may be conductive to the higher separation of the electron–hole pair, thus leading to the wonderful performance for the g‐C3N4‐NH3. Notably, the method has the merits of low cost, scalable production and environmental friendliness.

Poly(urea ester): A family of biodegradable polymers with high melting temperatures

Poly(urea ester)s are a class of polymers developed to bridge the gap between aliphatic and aromatic polyesters in which high melting temperature and biodegradability are in a contradiction. These new class of biodegradable polymers melt at higher temperature, and they can be simply prepared by condensation of urea, amino alcohols and dimethyl esters. Additional Supporting Information may be found in the online version of this article. Supporting Information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Photocatalytic reduction of CO2 by graphitic carbon nitride polymers derived from urea and barbituric acid

Conjugated carbon nitride nanosheets modified with barbituric acid (BA) were synthesized by a facile one-pot chemical condensation of urea. The obtained BA-modified carbon nitride samples were termed as CNU-BAX and were fully characterized by XRD, FTIR, XPS, NMR, EPR, FESEM, TEM, DRS, PL, BET and photocurrent measurements. The performance of the developed carbon nitride based semiconductors was investigated by applying them as polymeric photocatalysts for the reduction of CO2 under visible light illumination. Results revealed that the copolymerization of urea with BA co-monomer strongly alternated the physical and chemical properties of carbon nitride polymer by improving optical absorption and creating surface molecular heterojunction that promoted charge separation, and consequently the enhanced photocatalytic performance was achieved. Various reaction parameters were investigated and optimized for the reaction system, and we found that under the optimal reaction condition, the best sample (CNU-BA0.03) could effectively photocatalyze the CO2-to-CO conversion reaction with 15-fold-enhanced catalytic activity, compared to the non-modified sample derived from urea (named as CNU). Other typical comonomers were also selected to polymerize with urea to study the beneficial effect of copolymerization on the development of efficient carbon nitride based nanostructures for CO2 photoreduction.

Water-assisted production of honeycomb-like g-C3N4 with ultralong carrier lifetime and outstanding photocatalytic activity.

Graphitic carbon nitride (g-C3N4) is a visible light photocatalyst, limited by low activity mainly caused by rapid recombination of charge carriers. In the present work, honeycomb-like g-C3N4 was synthesized via thermal condensation of urea with addition of water at 450 °C for 1 h. Prolonging the condensation time caused the morphology of g-C3N4 to change from a porous honeycomb structure to a velvet-like nanoarchitecture. Unlike in previous studies, the photocatalytic activity of g-C3N4 decreased with increasing surface area. The honeycomb-like g-C3N4 with a relatively low surface area showed highly enhanced photocatalytic activity with an NO removal ratio of 48%. The evolution of NO2 intermediate was dramatically inhibited over the honeycomb-like g-C3N4. The short and long lifetimes of the charge carriers for honeycomb-like g-C3N4 were unprecedentedly prolonged to 22.3 and 165.4 ns, respectively. As a result, the honeycomb-like g-C3N4 was highly efficient and stable in activity and could be used repeatedly. Addition of water had the following multiple positive effects on g-C3N4: (1) formation of the honeycomb structure, (2) promotion of charge separation and migration, (3) enlargement of the band gap, (4) increase in production yield, and (5) decrease in energy cost. These advantages make the present preparation method for highly efficient g-C3N4 extremely appealing for large-scale applications. The active species produced from g-C3N4 under illumination were confirmed using DMPO-ESR spin-trapping, the reaction intermediate was monitored, and the reaction mechanism of photocatalytic NO oxidation by g-C3N4 was revealed. This work could provide an attractive alternative method for mass-production of highly active g-C3N4-based photocatalysts for environmental and energetic applications.

Strongly veined carbon nanoleaves as a highly efficient metal-free electrocatalyst.

Effective integration of one-dimensional carbon nanofibers (CNF) and two-dimensional carbon sheets into three-dimensional (3D) conductive frameworks is essential for their practical applications as electrode materials. Herein, a novel "vein-leaf"-type 3D complex of carbon nanofibers with nitrogen-doped graphene (NG) was prepared through a simple thermal condensation of urea and bacterial cellulose. During the formation of the 3D complex CNF@NG, the graphene species was tethered to CNF via carbon-carbon bonds. Such an interconnected 3D network facilitates both the electron transfer and mass diffusion for electrochemical reactions.

Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal‐Free Electrocatalyst

AbstractEffective integration of one‐dimensional carbon nanofibers (CNF) and two‐dimensional carbon sheets into three‐dimensional (3D) conductive frameworks is essential for their practical applications as electrode materials. Herein, a novel “vein‐leaf”‐type 3D complex of carbon nanofibers with nitrogen‐doped graphene (NG) was prepared through a simple thermal condensation of urea and bacterial cellulose. During the formation of the 3D complex CNF@NG, the graphene species was tethered to CNF via carbon–carbon bonds. Such an interconnected 3D network facilitates both the electron transfer and mass diffusion for electrochemical reactions.

Synthesis of 5-Cinnamoyl-3,4-dihydropyrimidine-2(1H)-ones

Two different approaches to the synthesis of 1-unsubstituted 5-cinnamoyl-3,4-dihydropyrimidine-2(1H)-ones have been developed. The first includes N(1)-protection of the starting 5-acetyl-3,4-dihydropyrimidine-2(1H)-one, further Claisen–Schmidt reaction, and cleavage of the protecting group. The second approach consists of one-pot condensation of urea, aldehyde, and cinnamoylacetone as dicarbonyl component. The 5-cinnamoylderivative synthesis starting from 5-acetyl-1,3-dialkyl-3,4-dihydropyrimidine-2(1H)-ones is also shown.

Synthesis and properties of alkylthiomethylated urea derivatives

Novel N,N′-bis[(alkylsulfanyl)methyl]ureas were prepared by three-component condensation of urea with formaldehyde and thiol in a NaOH solution, and alkylthiomethylated polymethyleneureas were prepared by the condensation of the same reactants in the presence of glacial acetic acid and without it. Polymethyleneureas efficiently sorb silver(I) from nitric acid solutions and palladium(II) from hydrochloric acid solutions. Alcoholsoluble N,N′-bis[(alkylsulfanyl)methyl]ureas inhibit carbon dioxide corrosion of St.3 steel.

Bi(OTf)3—a mild catalyst for the synthesis of difficult to obtain C-alkyl substituted glycolurils

We have employed Bi(OTf)3 (5 mol %) a catalyst not previously used in the synthesis of urea condensation products from α-dicarbonyl compounds, as a mild method for the synthesis of difficult to obtain glycolurils. There are limited synthetic examples of C-alkylated substituted glycolurils and even fewer carrying functionality on the alkyl substituent. We have successfully synthesised some challenging examples of glycolurils with functionality, which include norbornyl-based structures. The milder reaction conditions enables the isolation of defined intermediate diols and ureas in good yields, and glycolurils are possible, where they would not normally be available under conventional acid catalyzed conditions.

Diastereoselective synthesis of 4,5-dihydroxyimidazolidin-2-ones (-thiones) and their structure

Studies on the synthesis of 4,5-dihydroxyimidazolidin-2-ones (-thiones) based on the condensation of ureas or thioureas with glyoxal or 1,2-dioxo-1,2-diphenylethane showed high diastereoselectivity in the formation of racemates (trans-diastereomers) of 4,5-dihydroxyimidazolidin-2-ones (-thiones) and meso-forms (cis-diastereomers) of 4,5-dihydroxy-4,5-diphenyl-1,3-dialkylimidazolidine-2-thiones; plausible mechanisms of their formation were suggested. X-ray diffraction studies confirmed structures of diastereomers for separate examples of racemates and meso-forms of 4,5-dihydroxyimidazolidin-2-ones (-thiones).

Nitrogen Release from Slow‐Release Fertilizers as Affected by Soil Type and Temperature

Loss of N from fertilized agricultural soils is a serious problem that can negatively affect environmental quality. Nitrogen loss can be moderated by using slow‐release fertilizer (SRF) products (e.g., those created through condensation of urea and formaldehyde) in place of 100% water‐soluble N. Release of N from SRFs is affected by the soil environment. To evaluate soil effects on N release, we conducted an incubation study in which temperature and soil type were varied. Four SRFs were studied: liquid Nitamin 30L (L30), liquid Nitamin RUAG 521G30 (G30), granular Nitamin 42G (N42) (all from Georgia Pacific Chemicals, Decatur, GA), and granular Nitroform (NF) (Agrium Advanced Technologies, Loveland, CO). The fertilizers were incubated for 78 d at 20, 25, and 30°C in a sandy soil and at 25°C in a loamy soil. Differential N release kinetics of the N sources were determined by measuring NH4–N and NO3–N concentrations throughout the incubation. Net N released as a percentage of total N in the fertilizer was significantly affected by N source, temperature, time, and soil type. Increasing temperature increased net N release. The N release rate decreased in the order N42 > G30 > L30 > NF in the sandy soil and G30 > N42 > L30 > NF in the loamy soil. Overall, the release rates of these fertilizers were greater in the loamy soil. The N release characteristics determined in this study can help in the selection of the appropriate SRF source for crops grown under different soil and climatic conditions to improve N use efficiency and minimize N loss to the environment.

ChemInform Abstract: Facile Synthesis and Nitration of cis-syn-cis-2,6-Dioxodecahydro-1H,5H- diimidazo(4,5-b:4′,5′-e)pyrazine.

The title ring system was synthesized for the first time by acid-promoted condensation of ureas with 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine. Nitrosation and nitration of the polycycle occurs first at the piperazine nitrogens. Successive further nitration leads to tetra-, penta-, and hexanitro derivatives. X-ray crystallographic analysis of the tetra- and hexanitro derivatives established the cis-syn-cis configuration and an all-axial conformation for this ring system. Possible reasons for the stereoselectivity of the condensation reaction are discussed

Preparation and barrier properties of the microcapsules added nanoclays in the wall

AbstractPreparation of microcapsules applied to the fabrication of self‐healing composites has attracted a lot of attention. However, the leakage of core material from the microcapsule is a major problem in self‐healing microcapsules. Proper dispersion of layered silicates within the wall of microcapsule is a strategy for improving the barrier properties of the microcapsule. In the present study, poly(urea‐formaldehyde) (PUF) microcapsules containing dicyclopentadiene (DCPD) were prepared by in situ polymerization. For the preparation of UF/clay nanocomposite microcapsules, acid‐modified montmorillonite (H‐MMT) was used as an effective catalyst for the condensation of urea and formaldehyde, and the condensation polymerization in the galleries resulted in the delamination of the clay, as confirmed by TEM and XRD analysis. Scanning electron microscopy (SEM) was applied to observe the morphology of the microcapsules and the barrier property of microcapsules was investigated by thermal gravimetry (TG) analysis and mass release method. On comparison with conventional microcapsules (CMs), nanocomposite microcapsules (NCMs) have better barrier property. This can be attributed to the nanocomposite structure of the microcapsules, where nanosized montmorillonite dispersed in UF to decrease the core material cross‐over. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis–structure–performance relationship of cocondensed phenol–urea–formaldehyde resins by MALDI‐ToF and 13C NMR

AbstractMatrix assisted laser desorption ionization time of flight (MALDI‐ToF) mass spectrometry has consistently confirmed on a number of PUF resins that phenol–urea cocondensates exist in phenol–urea–formaldehyde (PUF) resins. A noticeable proportion of methylene‐linked phenol to urea cocondensates were detected in all the PUF resins tried, alongside methylene bridges connecting phenol to phenol and urea to urea. The PUF, PF, and UF oligomers formed were identified. Variations of the PUF preparation procedure did always yield a certain proportion of the mixed phenol to urea cocondensates. Their relative proportion was determined and related the synthesis procedure used. Comparison of the MALDI‐ToF results with a 13C NMR investigation showed that in a real PUF resin in which phenol to urea cocondensates were identified the methylene bridge NMR signal at 44 ppm, characteristic of phenol to urea unsubstituted model compound cocondensates, does not appear at all. This confirmed that this peak cannot be taken as an indication of the existence of phenol and urea condensation under actual resin preparation conditions. The peak indicating cocondensation in PUF resins in which the phenolic nuclei and urea are substituted appears instead at 54.7–55.0 ppm. Thermomechanical analysis has again confirmed that the resin gel times greatly accelerates with increasing urea molar content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008